Dihydrobenzoxazine components have been employed satisfactorily to produce prepregs, laminates, moulding materials, RTM (resin transfer moulding) systems, sealants, sinter powders, cast articles, structural composites parts, varnishes, surface coatings, electrical and electronic components by impregnating, coating, laminating or moulding processes.
Dihydrobenzoxazine components can easily be produced in several, well known ways by the reaction of bisphenols with a primary amine and formaldehyde, whereby the process can be carried out in the presence of solvents (see for example U.S. Pat. No. 5,152,993 or U.S. Pat. No. 5,266,695) or in the absence of solvents (see for example U.S. Pat. No. 5,543,516). Various hardeners such as novolacs, polyepoxides or polyamines are known to cure the dihydrobenzoxazine resin in order to obtain the valuable properties of the resins which make this class of thermosetting resins attractive.
EP 0 789 056 A2 describes a thermosetting resin composition with improved curability comprising dihydrobenzoxazines of polyphenols such as novolacs or bisphenol A and novolac phenolic resins. The composition is used as adhesive or for the manufacture of moulded articles, coatings, sealings, prepregs for printed wiring boards and metal-clad laminates with low water absorbance, improved non-flammability and high heat resistance. However, use of polyhydroxy functional novolacs as a hardener for the dihydrobenzoxazine resins lead sometimes to an undesirable high reactivity (low gel times) and, furthermore, to highly cross-linked resins, which generally are brittle.
WO 2006/035021 A1 describes bisdihydrobenzoxazines on the basis of phenolphthalein for the preparation of polymers, which show a high temperature stability and a good none-flammability. Polymerisation may be carried out in presence of catalysts, such as thiodipropionic acid, phenols or sulfonyl diphenol. However, the use of quaternary ammonium salts as catalysts is not mentioned in WO 2006/035021 A1.
WO 02/057279 A1 discloses phosphorous containing dihydrobenzoxazine resin composition comprising epoxy resins and quaternary ammonium salts as a possible hardener. However, the phosphorous containing dihydrobenzoxazine resin systems demonstrate a long gel time and a low reaction enthalpy which render said resin systems unsuitable for high reactive coating and moulding applications.
Especially for resin transfer molding processes it is desirable to be able to keep the thermosetting composition in a liquid or molten liquid state. Therefore, it is necessary that at this stage of the process the thermosetting composition does not cure rapidly. However, once the article is shaped it is desired that once the temperature is increased the thermosetting composition cures rapidly.